In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on the transmission path between a transmitter and a receiver can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If there is an obstacle with sharp irregularities on the transmission path, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves will interact with each other and result in multipath fading.
A multipath signal combiner is one of the methods to deal with the multipath-fading problem. For each path of a multipath-fading signal, there is a corresponding component signal. A multipath signal combiner in a receiver is to combine all the significant component signals according to their corresponding signal strengths. On average, a multipath signal combiner can provide a signal more stable and stronger than each individual component signal and therefore improves the system performance.
A demodulator of a coherent receiver has to remove both frequency error and phase error to recover data. Ordinarily, there are a frequency corrector and a phase rotator. The frequency corrector is for removing the frequency error so that the frequency error remaining after correction does not exceed a few percent of the symbol rate. The phase rotator is for getting rid of the residual frequency error and the phase error.
Nowadays one of the most common burst communication systems is packet-switched communication system. As a burst communication system, a packet-switched communication system places unusual demand on a carrier recover circuit especially when the transmitted data rate is substantially high. The data received at a receiver could from a transmitter for a short length of time and then from another different transmitter for another short length of time. The different bursts of data come from different transmitters and have no phase coherence from one burst to the next in most situations. In order to achieve good efficiency, only a very small portion of each burst can be devoted to carrier recovery in a packet-switched communication system. Usually, this very small portion is located at the beginning of each burst. The symbols in the very small portion are called preamble symbols.
When symbol rate is so high that the combination of Doppler frequency spread and frequency offset is no more than a few percent of a symbol rate, it is possible to use only a phase rotator to correct both the frequency error and phase error.
FIG. 1 is the essential portion of a baseband multipath RAKE receiver with the capability of frequency correction and phase correction. Suppose that there are at most K significant multipath components. Multipath splitter 105 splits the received complex signal Rin into K complex component signals. Each of the delay devices 1101 to 110K delays one of the K complex component signals for a different amount of time. Each of the multipliers 1151 to 115K scales the output complex signal from one of the delay devices 1101 to 110K by a corresponding weight from controller 130 respectively. Each of the phase rotator devices 1201 to 120K rotates the output complex signal from one of the multipliers 1151 to 115K by a corresponding phase from the controller 130 respectively. Adder 125 adds the output signals from the phase rotator devices 1201 to 120K together to generate a summation signal. Decision circuit 130 makes decision on the transmitted symbol from the summation signal. Controller 135 collects information from various devices and generates necessary control and timing signals for relevant devices such as the delay devices 1101 to 110K, the multipliers 1151 to 115K and the phase rotator devices 1201 to 120K.
FIG. 2 shows the structure of a conventional phase rotator. The desired phase adjustment θ is fed to ROM (read only memory) device 205 to obtain corresponding signals sin(θ) and cos(θ). The complex input signal of the phase rotator consists of a real signal Iin and an imaginary signal Qin. Multiplier 2101 multiplies the real signal Iin by cos(θ) to obtain the first product and multiplier 2102 multiplies the real signal Iin by sin(θ) to obtain the second product. Multiplier 2103 multiplies the imaginary signal Qin by cos(θ) to obtain the third product and multiplier 2104 multiplies the imaginary signal Qin by sin(θ) to obtain the fourth product. Adder 2151 subtracts the fourth product from the first product to obtain a signal Iout and adder 2152 adds the second product to the third product to obtain a signal Qout. The output signal of the phase rotator is a complex signal (Iout, Qout).
Mathematically, one can obtainIout+jQout=(Iin+jQin)·elθ=(Iin·cos(θ)−Qin·sin(θ))+j(Iin·sin(θ)+Qin·cos(θ))  (1)
Since sine and cosine functions are nonlinear functions and difficult to calculate them with enough precision on real time, usually they are pre-calculated and stored in ROM as shown in FIG. 2 and therefore a lot of hardware will be consumed.
Therefore, it would be desirable to eliminate the evaluation of the nonlinear function sin(θ) and cos(θ) in a phase rotator.